The present invention relates to hydraulic fluids. More in particular, the invention relates to ester-based hydraulic fluids having improved low temperature properties.
Hydraulic systems are used in a wide variety of mechanical equipment, such as automobiles, trucks, cranes, trains, other transport equipment, agricultural equipment, ships and marine equipment (all mobile systems) and non-mobile systems such as in factories and railways. Such hydraulic systems contain a hydraulic fluid, which can be based on a petrochemical fluid (traditional) or an ester/oleochemical basis (more environmentally acceptable).
In a number of applications, these hydraulic systems are subjected to low temperatures, of either the environment or the hydraulic system itself. This is especially the case when hydraulic systems are used in countries near or within the arctic circles. It is a known fact that a number of properties of hydraulic fluids change upon changing temperature of the hydraulic fluid. It is also known that low temperatures have generally an adverse effect on a number of properties of the hydraulic fluid, such as the pourpoint and the (dynamic) viscosity being too high and an insufficient low temperature stability, etcetera.
For hydraulic fluids based on petrochemical fluids, a number of solutions have been proposed to improve some of the low temperature properties. For example by the use of additives such as pourpoint depressants. However, up till now, it is known that such additives cannot be used in hydraulic fluids based on esters/oleochemicals, because they are either incompatible with the esters/oleochemicals, or such additives have an insufficient effect (the desired effect being good properties at low temperatures).
Hence, there was a need for an ester/oleochemical-based hydraulic fluid having improved low temperature properties, when compared with conventional ester/oleochemical based hydraulic fluids. The xe2x80x9cester/oleochemical based fluidsxe2x80x9d are herein to be understood as to be fluids, suitable for application as a hydraulic (base) fluid for use in a hydraulic system, in which at least the major part (i.e. more than 50% by weight) is composed of an ester of a polyol and a carboxylic acid. It is a further object of the invention that the ester/oleochemical based hydraulic fluid still has satisfactory properties at xe2x80x9cnormal-usexe2x80x9d temperatures, in addition to the improved low temperature properties. An important property relating to xe2x80x9cnormal usexe2x80x9d temperatures is the kinematic viscosity at 40xc2x0 C. and 100xc2x0 C. By improved low temperature properties is meant that the properties of the hydraulic fluid are improved in at least one aspect, preferably being low temperature stability.
By low temperature stability is meant that the liquid still has suitable properties after being kept at a low temperature for a considerable amount of time (a number of days). A way of quantifying low temperature stability is by the test method according to ASTM D2532. In short, the method consists of measuring the (kinematic) viscosity of the liquid, after it has been kept at xe2x88x9230xc2x0 C. for 168 hours. The viscosity so measured should then be below a specified limit.
It has now been found that the above objectives can be met by a composition comprising an ester of a polyol and a mixture of fatty acids, wherein at least a part (I) of the esterified fatty acids has a chain length of 5-12 carbon atoms and another part (II) of the esterified fatty acids has a chain length of 16-22 carbon atoms, and wherein the composition has a viscosity of 7000 mm2/s or less, when tested according to ASTM (D2532). Such an ester is often referred to as a mixed ester, meaning that (on average) each ester molecule contains at least two different carboxylic acid moieties. Said short chain fatty acids as well as long fatty acids can be straight chain or branched chain (or mixtures thereof).
Preferably, the composition has a viscosity of 5000 mm2/s or less, when tested according to ASTM (D2532).
For a combination of good low- and high temperature properties, it is preferred that the ratio short chain fatty acid:long chain fatty acid in the mixed esters should be between 2:1 and 1:20 by weight. It was found that for many cases a relatively small amount of short chain fatty acids is needed for obtaining the desired effect, and hence, it is even more preferred that the above referred ratio is between 1:1 and 1:10 by weight.
Although the compositions according to the invention are preferably free of additives such as an emulsifier or a pourpoint depressant, specific additives such as an anti-oxidant (e.g. aminic or phenolic type anti-oxidants) viscosity index (VI) improver (e.g. polymethacrylate compounds), an anti-wear compound (e.g. dithiophosphates, calcium sulphonates, barium sulphonates) and an anti-foam compound (e.g. modified dimethyl polysiloxanes) may be added to the compositions.
The compositions according to the invention preferably comprise esters of polyols selected from the group consisting of TMP (trimethylol propane), PE (pentaerythritol), NPG (neopentylglycol), di-TMP, tri-TMP, di-PE, tri-PE. A most preferred polyol is TMP.
The polyols are preferably esterified with mixtures comprising short chain fatty acids having branched or straight chain C8 or C10 fatty acids, or mixtures thereof. For the long-chain part, it is preferred that these comprise oleic acid or isostearic acid.
Preferably, the esters according to the invention have an acid value of less than 5.0, preferably less than 1.0 mg KOH/g.
Preferably, the composition to be used as a hydraulic fluid comprises at least 75%, more preferably, at least 85% by weight of the mixed esters as above defined. Most preferred is a hydraulic fluid comprising at least 95% by weight of the esters as defined above.
Although for some cases the compositions according to the invention can be used as such in a hydraulic system, it is also possible that such compositions are used in the manufacture of hydraulic fluids, by e.g. compounding them with other fluids or additives.
A further embodiment of the invention is the use of the compositions according to the above in hydraulic equipment. Such hydraulic equipment may be part of a mobile system.
The invention further comprises a method for the manufacture of hydraulic fluids, comprising the esterification of a polyol with a mixture of fatty acids, wherein that at least a part of the fatty acids has a short chain length (5-12 carbon atoms) and another part of the fatty acids has a long chain length (16-22 carbon atoms), and wherein the fatty acid ester is present in the composition in an amount of at least 75% by weight, based on the total composition, and optionally mixing the resulting mixed ester with other components. Said short chain fatty acids as well as long fatty acids can be straight chain or branched chain (or mixtures thereof).
The invention is further illustrated by the following examples, which are not to be understood as limiting the invention thereto.